1. Field of the Present Invention
The present invention relates, generally, to laser scanning systems, and, more particularly, to laser scanning systems that utilize at least one diffractive optical element to direct laser light beams through a scanning region.
2. Brief Description of the State of the Art
Laser scanning systems utilize a laser light source (such as a visible laser diode (VLD)) and optical elements to direct laser light beams through a scanning region, and optical elements, photodetector(s) and analog/digital processing circuitry to collect, capture and analyze the returning (i.e. incoming) laser light beams reflecting off light reflective surfaces (e.g. product surfaces, bar code symbols, etc).
One class of laser scanning systems (hereinafter referred to as “Diffractive-Based Laser Scanning Systems) utilize one or more diffractive optical elements to direct the outgoing laser light and/or collect the incoming laser light. A diffractive optical element (referred to below as a “DOE”) is an optical structure that operates on the principle of diffraction—it breaks up an incident laser light beam into a large number of waves, which recombine to form completely new waves. A DOE can function as a grating, lens, aspheric or any other optical element. Diffractive-based laser scanning systems include holographic laser scanning systems that use one or more multi-faceted holographic optical elements to direct the outgoing laser light through the scanning region and collect the incoming laser light for capture by the photodetector(s).
As shown in FIG. 1, an exemplary diffractive-based laser system 100 employs a laser light source 101 (such as a solid state VLD) that emits laser light beams (denoted I) having a characteristic wavelength. An optical subsystem 103 directs portions (denoted I) of these laser light beams into a scanning region 105. The returning (i.e., incoming) laser light beams (denoted I″) from the scanning region 105, which reflect off light reflective surfaces in the scanning region 105, are collected by the optical subsystem 103 and portions (denoted I′″) of the returning laser light beams are directed to photodetector(s) 107 and signal processing and control circuitry 109 that capture and analyze the returning laser light beam portions to identify properties (such as bar code symbols, spatial dimensions, spatial profiles, and velocity) of the surfaces within the scanning region. The optical subsystem 103 utilizes at least one diffractive optical element (DOE) in directing the laser light beams I′ into the scanning region 105, collecting the returning laser light beams I″, and/or directing the portions of the returning laser light beams I′″ to the photodetector(s) 107.
Laser light sources, such as solid-state VLDs, typically exhibit mode switching, which manifests itself as a shift in the characteristic wavelength of light emitted from the laser light source. Mode switching can occur at frequencies ranging from a few hertz to several hundred kilohertz. In systems using VLDs, mode switching is related to the temperature of the VLD. More specifically, as the temperature of the VLD varies, the physical dimensions and characteristics of the semiconductor material of the VLD change, thereby favoring operation at various wavelengths (i.e., modes). In addition, mode switching can be induced by optical feedback into a laser source (e.g., VLD).
In diffractive-based laser scanning systems, including holographic laser scanning systems, mode switching of a laser light source can potentially cause unwanted variations in the amplitude and direction of light directed through the scanning region, as well as unwanted variations in cross-sectional dimensions and beam shape of the laser scanning beams. If such variations are significant, the light beams entering the scanning region may not move uniformly through the scanning region (as designed), instead jumping rapidly about its expected position. This results in an effectively larger “spot” size of the light beam at its focal point in the scanning region, which may lead to unwanted distortion and signal processing errors, for example, errors in the resolution of the bars and spaces of scanned code symbols and, often, intolerable symbol decoding errors.
Such variations result from the optical characteristics of the diffractive optical elements used therein. More specifically, the amplitude and direction (and other optical properties) of the diffracted light beam output from a diffractive optical element is sensitive to wavelength of the incident beam. In other words, the amplitude and direction of the diffracted light beam output from the diffractive element is a function of wavelength of the incident beam. Thus, variations in wavelength of the light beam incident on such diffractive optical elements can cause unwanted variations in amplitude and direction of the diffracted beam, which may result in non-uniform movement and distortion of the light beam directed through the scanning region and unwanted signal processing errors (for example, errors in the resolution of the bars and spaces of scanned code symbols and, often, intolerable symbol decoding errors) as described above.
Thus, there is a great need in the art for an improved diffractive-based laser scanning system that minimizes the effects of mode switching (shift in characteristic wavelength) of laser light sources employed therein, while avoiding the shortcomings and drawbacks of prior art diffractive-based scanning systems and methodologies.